This invention relates to a look washer assembly adapted for use between a threaded fastener nut (or the head portion of a bolt) and the surface of a workpiece that is being fastened to another workpiece. The lock assembly comprises two similarly configured lock washers, each having a flat face and a serrated face formed with a series of circumferentially-spaced, wedge shaped cam surfaces. The serrated faces of the individual lock washers are engaged together, such that rotation of the fastener nut in the loosening direction, as by a vibrating condition, causes the serrated faces to slide against each other such that one of the washers becomes wedged with respect to the other washer, filling any space between the washers caused by the vibrations.
U.S. Pat. No. 3,263,727 to A. Herpolsheimer shows a lock washer assembly comprising two individual lock washers arranged with cammed faces in mutual engagement. The outer faces of the lock washers have serrations which "must dig into the member 36 and nut 38 to depths wherein they will not be disengaged as the bolt elongates". (Col. 3, lines 48-51) designed to become embedded in the opposed surfaces of a workpiece and a fastener element (i.e. a threaded nut or the head of a threaded bolt). The intent is to have the teeth "bite" into the associated surface to prevent slippage between the washer and the workpiece or threaded element. Herpolsheimer indicates that the projecting teeth exert a cutting action on the opposed surface to remove small portions of the metal. The teeth plow into the metal surface.
One disadvantage of such toothed structures is that the surface on the workpiece or fastener element has to be relatively soft in order for the teeth to exert the desired cutting action.
Another disadvantage of the toothed structure is that they form gouges in the workpiece. Such gouges can form stress riser cracks leading to fatigue failure of the workpiece.